Vibratory angular rate sensing system

ABSTRACT

.[.A vibratory angular rate sensing system may have one pair of tines forming an angle of about 60 degrees, resulting from the crystalline orientation of the Z-cut quartz wafer of the system. In one configuration each of the tines is provided with a mass offset from the axis of the associated tine, a pivot extends through the plane of symmery of the tines and may be connected to a dummy reaction mass. The resonant system is suspended from a mounting frame by a pair of suspension bridges. A second embodiment of the invention features two groups of tines, each group including two pairs of tines arranged in the form of a cross. Again, an offset mass is associated with the free end of each tine and each group of tines is secured to its frame by a suspension bridge..]..Iadd.Vibratory angular rate sensing system having a support structure and a driven fork lying in a plane and having an axis of symmetry. The fork has first and second spaced apart tines. A mouting is provided for mounting said fork on said support structure. Energy is coupled into the driven fork to cause vibratory motion of the tines of the driven fork in the plane of the driven fork. A torsionally resonant member is coupled to the fork. A pickoff is provided which is isolated from the support structure for sensing motion of the torsionally resonant member to provide a measure of the input angular rate about the axis of symmetry of the driven fork. .Iaddend.

.Iadd.This is a continuation of application Ser. No. 859,474 filed May2, 1986, abandoned. .Iaddend.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application may be considered to be an improvement and anextension of the principles of some of the present applicants' priorapplication entitled, "Angular Rate Sensor System," to Alsenz, et al.,Ser. No. 06/321,964, filed Nov. 16, 1981. The present application isassigned to the same assignee as is the prior copending application.

The present application is also related to a copending applicationentitled, "Vibratory Angular Rate Sensor System," to Staudte, Ser. No.06/572,783, which is also assigned to the same assignee as is thepresent application. The Staudte application has been filed concurrentlywith the present application and may be considered to be a differentembodiment for the same purposes; that is, for minimizing undesiredvibrations which may cause undesired noise.

BACKGROUND OF THE INVENTION

The angular rate of motion of a craft is an essential input for allnavigational and inertial guidance systems. Such systems are usedconventionally for aircraft, spacecraft, ships, or missiles. The sensingof the angular rate of motion is presently accomplished by means of agyroscope.

Gyroscopes, however, have various disadvantages. They must be built toextremely high accuracies and may have drift rates of fractions of adegree per hour. Due to the expense of building them, they are verycostly; they are physically large and heavy. They must be frequently andprecisely maintained, for the reason that critical movable elements,such as bearings, may change with time. They may also be damaged by evenlow levels of shock and vibration. This, in turn, may cause an increaseof unknown size in the drift rate, occurring at unknown times.

Because gyroscopes are sensitive to the effects of shock and vibration,they frequently have heavy mounting configurations to protect them,which also are expensive.

SUMMARY OF THE INVENTION

It will, accordingly, be obvious that it is desirable to replace agyroscope by some other device which is less expensive and which iscapable of measuring angular rates, thereby measuring the attitude of avehicle or craft. In accordance with the present invention, this isaccomplished by a balanced resonant sensor. Such a sensor isrepresented, in accordance with the present invention, by a tuning fork.The tuning fork should be substantially mechanically temperature-stable,have low internal friction, and follow Hook's Law. According to Hook'sLaw, the strain of an elastic body is proportional to the stress towhich the body is subjected by the applied load (the strain, however,must be within the elastic limit of the body), and the body will returnto its original shape when the stress is removed.

Preferably, but not necessarily, the tuning fork consists of quartz.However, other piezoelectric materials may be used, such as syntheticcrystals; for example, ethylene diamine tartrate (EDT), dipotassiumtartrate (DKT) or ammonium dihydrogen phosphate (ADP). Non-piezoelectricmaterials may be used within an electromagnetic drive.

According to the present invention, the angular rate sensing system ofthe invention is carved from a plate of Z-cut quartz, quartz being thepreferred material. Since the plate has to be chemically etched orotherwise cut, for example by a laser beam or similar techniques, theorientation of the wafer is important, because etching along the Z-axis(that is, along the thickness of the wafer) is considerably faster andeasier. Since a Z-cut quartz wafer has trigonal symmetry, the anglebetween, for example, a plus X and the next adjacent minus X directionis 60 degrees, the tines are oriented at such an angle of 60 degrees. Inother words, the crystalline orientation permits a three-fold redundantchoice of axis.

The structure consists basically of a frame within which is suspended,by two suspension bridges, a pair of tines, preferably at a 60 degreeangle to each other. A pivot extends through the symmetry axis of thetines and is secured to what may be called a dummy reaction mass. Aseparate mass is secured to the free end of each tine but is offset fromthe axes of the tines.

The tines are vibrated, for example, electrically through electrodesdriven by a drive oscillator substantially at the resonant frequency ofthe system, which is determined by the reaction mass, the two masses,the times, and the base of the tines. When the entire sensor is rotatedin inertial space about the axis of symmetry, the masses, due to theirmotion relative to the rotation, will experience a force at right anglesto their motion, in accordance with the Coriolis effect. The Coriolisforce on the masses is such as to produce a torque on the sensor. Thisoutput torque strains a torsion member having electrodes disposed topick up the piezoelectric charge created by the strain. The electrodesmay, for example, consist of a thin gold film which has beenvapor-deposited and may then be photolithographically etched through asuitable mask.

A second structure is also disclosed which includes two groups of tines,each group consisting of two pairs of tines disposed substantially inthe shape of an X. One group of tines is driven in phase opposite to theother.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of one configuration of the vibratorysensing system of the invention, including one pair of tines, the waferorientation being shown adjacent to the structure;

FIG. 2 is a schematic picture of the quartz crystalline orientation;

FIG. 3 is a cross-sectional view taken on lines 3--3 of one of the tinesto show the position of the drive electrodes and a drive oscillator withthe wafer orientation shown with respect to the section;

FIG. 4 is a similar cross-sectional view taken on lines 4--4 of thepivot of the system, showing two pairs of output electrodes connected toan output circuit, the crystal orientation also being shown adjacent toFIG. 4;

FIG. 5 is a plan view of another preferred modification of theinvention, comprising two groups of two pairs of tines each;

FIG. 6a is a cross-sectional view taken along lines 6a--6a of FIG. 5through one of the tines of one pair of the first group of tines andshowing two pairs of electrodes disposed thereon;

FIG. 6b is a cross-sectional view taken along lines 6b--6b of FIG. 5 ofa tine of a pair of tines of the second group, to show the four driveelectrodes for a pair of the second group of tines;

FIG. 6c is a schematic circuit diagram showing how the first set of fourelectrodes is driven in phase opposition to the second sets of driveelectrodes of the second group of tines; and

FIG. 7 is a cross-sectional view taken along lines 7--7 of FIG. 5showing two pairs of electrodes for obtaining the output signal and anoutput circuit connected thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and particularly to FIGS. 1 through 4,there is illustrated a first embodiment of the present invention. Theembodiment includes a mounting frame 10 which preferably consists of aZ-cut wafer of quartz. The wafer orientation has been shown adjacent tothe frame 10 with the +X, +Y, and +Z directions. FIG. 2 illustrates thetrigonal symmetry of the quartz crystal. It will be evident that the +Xdirection shown at 11 and the next adjacent -X direction 12 form anangle of 60 degrees with each other, etc. Accordingly, the two tines 14and 15 of the system are disposed substantially at an angle of 60degrees, starting from their base 16.

There is also provided a pivot 17 which extends along the symmetry axisbetween the two tines 14 and 15. One end of the pivot 17 is secured towhat may be called a dummy reaction mass 20. Furthermore, a mass 21 issecured to the tine 14 at an offset angle with respect to the axis ofthe tine 14. A like mass 22 is similarly connected to the other tine 15.

The entire system, including the tines 14, 15, the base 16, the masses21, 22, the pivot 17, and the dummy mass 20 are disposed within anopening 25 of the frame 10. Furthermore, this system is secured to theframe 25 by a pair of suspension or support bridges 26 and 27. There arefurther provided pivot support or bridge extensions 28 and 30 whichconnect the system to the two suspension bridges 26 and 27.

It will be understood that the two suspension bridges 26 and 27 are theonly means supporting and securing the resonant system to the frame 10.

By way of example, the short side of the rectangular frame 10 may have alength of 0.400 inch, and the long side may have a length of 0.575 inch,the wafer having a thickness of 0.020 inch. Of course, these dimensionsmay change according to practical requirements, or the properties of thematerials used.

The resonant system is driven by a first set of two drive electrodes 31and 32 and a second set of drive electrodes 34 and 35, as shown in FIG.3. The electrodes 31, 32 and 34, 35 are connected to each other andacross a drive oscillator 36. It will be understood that both drivetines 14 and 15 are excited as illustrated in FIG. 3.

As shown by the adjacent wafer orientation, the electrodes 34, 35 aredisposed along the Z-axis and the electrodes 31, 32 are arranged alongthe X-axis. The frequency of the drive oscillator 36 should beapproximately that of the resonant system including the reaction mass20, the masses 22, the tines 14, 15 and the base 16.

The output signal is obtained from the pivot 17, again by means of twopairs of electrodes as shown in FIG. 4. There is provided a first pairof electrodes 40, 41, and a second pair of electrodes 42, 43. Both pairsof electrodes are arranged along the Z-axis, as shown by thecross-section of FIG. 1. Electrodes 41 and 42 are connected together,while electrodes 40 and 43 are connected together and across an outputcircuit 45. The output circuit may be entirely conventional, to derive asignal representative of the input force.

The system responds only to rotation in the input plane and not to otherrotations or to linear acceleration such as caused by gravity. On theother hand, the system requires an extremely precise mass balance.

It should be noted that the voltage-strain relationship varies with theorientation of the surface relative to the geometric axis system, asshown in FIGS. 1 and 2. A balance of the system is achieved by designingthe structure in approximately the shape of a tuning fork. Also, aportion of the surface of the structure of FIG. 1 may be covered with agold film which may be removed partially by a laser, or by etching, toobtain complete mass balance. This will provide an inherent geometricdrive balance and a reaction inertia to the output torque whichsubstantially avoids transferring energy to the mounting frame. This, ofcourse, makes the system more efficient and also immune to environmentalinfluence.

The system resonates at a frequency which is related to the inertia ofthe masses of tines and the stiffness of the tines. Because quartzinherently has a piezoelectric effect, electrical excitation of thesystem electrodes results in a strain of the tines.

It should be noted that the polarities of the electrodes, as shown inFIG. 3, are such that the tines resonate in mechanical opposition. Inother words, the ends of the tines at one time approach each other andin some instant later move away from each other.

Furthermore, since the stiffness of the tines in the perpendicular or Zdirection is much higher than that in the X or Y direction, thestructure resonates only within the plane of the tines; that is, in theX-Y plane.

The system operates as follows: A constant rotation rate in inertialspace about the input axis produces a Coriolis torque couple, attemptingto rotate the structure in phase with the mass-drive velocity. The inputaxis may be defined by the intersection of the plane of the tines andthe plane of symmetry of the tines. In other words, it may be consideredto pass through the center of the pivot 17. The torque is transmitted tothe pivot 17.

The torsional stiffness of the pivot and the rotational inertia of themass and the tines about the input axis provide a rotational resonantsystem. The drive frequency is established at or near the resonantfrequency of the system.

The pivot strain resulting from the Coriolis torque is enhanced by afactor as much as the Q of the pivot system.

The output electrodes 40 to 43 disposed on the pivot 17 pick up areciprocating charge of amplitude proportional to the torsional strain.This, in turn, is proportional to the Coriolis torque, which, finally,is proportional to the input inertial rotation rate.

As explained hereinabove, the drive tines 14 and 15 are not disposedparallel to each other, but form an angle of approximately 60 degrees.This enhances the piezoelectric coupling of the drive electrodes. Theinherent axial thrust at the fundamental drive frequency can cause largefundamental and small harmonic thrust components, which can cause energyloss to the mounting frame 10. However, since the masses 21, 22 move inarcs controlled by an appropriate offset thereof from the time axes, thefundamental thrust component is substantially cancelled.

This energy loss can be further minimized by the configuration of FIG.5, where identical pairs of tines are disposed mechanically and drivenin phase electrically, so that all thrust components are substantiallycancelled. On the other hand, in the configuration of FIG. 1 the masses21 and 22 form part of the tines 14, 15, to cancel only the first orderthrust component.

Referring now to the second embodiment of the present inventionillustrated in FIG. 5, there is again provided a mounting frame 50 ofrectangular outline, having a rectangular opening 51 within which thevibrating system is disposed. The vibrating system includes a firstgroup of two pairs of tines 53 and a second group of vibrating tines 54.The first group 53 has a first pair of tines 55, 56 and a second pair oftines 57, 58. The two pairs of tines 55, 56 and 57, 58 each form, again,an angle of about 60 degrees, with the two pairs of tines havingapproximately the configuration of an X. Offset masses 60, 61 areassociated in an offset direction with the two tines 55, 56 similar tothe masses 21, 22 of FIG. 1, and similarly offset masses 62, 63 aresecured, respectively, to the tines 57, 58. A pivot 65 extends throughthe axis of symmetry of the first group of tines 53.

The second group of tines 54 is identical to that of the first group oftines, and corresponding elements have been designated with the samereference numbers, primed, as the first group of tines.

It will be noted that the system of FIG. 5 does not require a reactionmass, as does that of FIG. 1. The crystalline orientation is the same asthat of the system of FIG. 1.

The first group of tines 53 is made integral with the frame 50 by asuspension bridge 66, while 66' designates the respective suspensionbridge for the second group of tines 54. The two suspension bridges 66and 66' pass through the center point 67 or 67' of each two pairs oftines.

The structure of FIG. 5 may have a width of 0.500 inch in the Ydirection, a length of 1.080 inch in the X direction, and a thickness inthe Z direction of 0.020 inch.

Referring now to FIGS. 6a and 6b, there are illustrated the driveelectrodes for each group of tines 53 and 54 (shown in FIG. 5). In ordernot to confuse the drawings only one tine 55 and 55' of each group isillustrated in FIGS. 6a and 6b.

The tines 55, 56 and masses 60, 61 are driven in the same manner as thetines 57, 58 and masses 62, 63. To this end, a pair of electrodes 70, 71and 72, 73 is provided on each of the tines. The electrodes 70, 71 areconnected together, as are the other electrodes 72, 73, to form outputleads 82.

Similarly, as shown in FIG. 6b, another pair of electrodes 74, 75 and alast pair of electrodes 76, 77 are disposed on the tines 55', 56', and57', 58', each pair of electrodes being connected together as shown andhaving output leads 83. As shown in FIG. 6c, a drive oscillator 80 hasits terminals connected by leads 82 across the first two pairs ofelectrodes 70, 71 and 72, 73, and by leads 83 across the second twopairs of electrodes 74, 75 and 76, 77 opposite in phase to the firstpairs. As a result, the tines of the group 54 are driven in phaseopposition to the tines of group 53.

Equal but opposite Coriolis torques are generated by the two tinegroups, so that the torsion on the pivot 65 is balanced. The net resultis that no net force is felt by the mounting frame 50.

The output signal is picked up from the pivot 65 by the electrodesillustrated in FIG. 7 in a manner similar to that shown in connectionwith FIG. 3.

It should be especially noted that the support bridges 26 and 27 of FIG.1 and the bridges 66 and 66' of FIG. 5 have a flexural and torsionalstiffness which is small compared to that of the respective pivot 17 andtines 14, 15, or the pivot 65 and tines 55, 56 and 57, 58 and thecorresponding tines of group 54. Furthermore, the support masses are sosmall that the flexural resonant frequency is substantially the same asthe drive and torsion frequency. As a result, the supports such as 26,27 and 66, 66' present a resonant load on the rotational motions of thesensor which is even less than the static stiffness by a factor of the Qof the support resonance. This, of course, results in a high isolationof the sensor from the outside environment, which is very desirable.

What is claimed is:
 1. A vibratory angular rate sensing systemcomprising:(a) a wafer of crystalline quartz having piezoelectricproperties and forming a .[.substantially rectangular.]. mounting.[.frame and having a substantially rectangular central opening.].; (b)a .Iadd.fork having a .Iaddend.pair of tines extending at apredetermined acute angle .[.within said opening.]. .Iadd.from an originand having an axis of symmetry.Iaddend.; (c) .Iadd.and .Iaddend.a baseinterconnecting said tines near their origin; (d) .[.a first and asecond suspension bridge, each being secured at its ends to.]..Iadd.bridge means supporting said fork on .Iaddend.said mounting.[.frame.].; (e) a .[.pivot.]. .Iadd.torsion member .Iaddend.extending.[.between.]. .Iadd.along .Iaddend.the axis of symmetry of said.[.tines.]. .Iadd.fork .Iaddend.and through said base.[., said pivot.]..Iadd.and .Iaddend.being secured .[.near both ends by said suspensionbridges.]. .Iadd.to said bridge means for forming bridges.Iaddend.; (f)a reaction mass secured to said .[.pivot at its end extending beyond thewide opening of said tines.]. .Iadd.torsion member.Iaddend.; (g) a pairof masses, each being secured to the free end of one of said tines, saidtines having large fundamental and small harmonic thrust components,.[.and.]. said pair of masses being each disposed with the center ofmass offset from the axis of the associated tine.[.,.]. whereby thecenters of each mass move in arcs having substantially no thrustcomponent; (h) means .[.including a first pair of electrodes secured tosaid tines.]. for driving said .Iadd.fork to cause said .Iaddend.tines.Iadd.to vibrate .Iaddend.substantially at their resonant frequency.[.determined by said pair of masses, said tines and said base.].; and(i) .[.a second pair of electrodes secured.]. .Iadd.means coupled.Iaddend.to said .[.pivot.]. .Iadd.torsion member .Iaddend.for pickingup an output signal representative of torsion which produces a shearstrain resulting in an amplitude variation of .[.the.]. .Iadd.an.Iaddend.electric field.
 2. A system as defined in claim 1 wherein saidreaction mass has substantially the same moment of inertia as do saidmasses, said base and said tines, combined.
 3. A system as defined inclaim 1 wherein said water is of a Z-cut quartz plate and the anglebetween said tines is substantially 60 degrees.
 4. A system as definedin claim 3 wherein said .[.second pair of electrodes.]. .Iadd.meanscoupled to said torsion member .Iaddend.picks up a reciprocating changeof amplitude proportional to the torsional strain and hence to theCoriolis torque acting on said system.
 5. A system as defined in claim 1wherein said .[.bridges have.]. .Iadd.bridge means has .Iaddend.aflexural and torsional stiffness which is small compared to that of saidtines and of said .[.pivot.]. .Iadd.torsion member.Iaddend..
 6. Avibratory angular rate sensing system comprising:(a) a wafer ofcrystalline quartz having piezoelectric properties and forming asubstantially rectangular mounting frame having a substantiallyrectangular central opening; (b) a first group of two pairs of tines,each pair of tines forming the same predetermined angle between eachother as does the other pair, to provide substantially a configurationof a first cross; (c) a second group of two pairs of tines arranged inthe form of a cross and substantially like said first group and withsubstantially the same angles between each pair of tines of said secondgroup, said two groups being disposed within said opening of said frameand along the long sides of said mounting frame; (d) a first two pairsof masses, each being secured to the free ends of said tines of saidfirst group and a second two pairs of masses, each being secured to thefree ends of said tines of said second group, said masses being eachdisposed with the center of mass offset from the axis of the associatedtine; (e) a pivot passing through the axis of symmetry of said tines andpassing through the centers of said groups; (f) a pair of suspensionbridges, each extending between said frame and supporting one group oftines at the center thereof and passing through an associated one ofsaid group of tines; (g) means including two pairs of electrodes securedto said groups of tines for driving each pair of tines of a group out ofphase with respect to each other; and (h) means including a further pairof electrodes for picking up an output signal from said pivot.
 7. Asystem as defined in claim 6 wherein said wafer is of a Z-cut quartzplate and the angle between said pairs of tines is substantially 60degrees.
 8. A system as defined in claim 6 wherein said means fordriving generates a wave at a frequency substantially equal to theresonant frequency determined by said masses and said tines, the waveapplied to said first group of tines being out of phase with respect tothat applied to said second group of tines, whereby the thrust componenton said two pairs of tines of each group are equal and opposite, therebysubstantially cancelling any force which might otherwise react on saidframe.
 9. A system as defined in claim 6 wherein said bridges have aflexural and torsional stiffness which is small compared to that of saidtines and of said pivot. .Iadd.
 10. In a vibratory angular rate sensingsystem, a wafer of crystalline quartz having piezoelectric propertiesand providing a mounting, a fork having a pair of tines extending at apredetermined acute angle from an origin about an axis of symmetry andhaving a base interconnecting the pair of tines near their origin, meansfor forming a bridge supporting and isolating said fork on saidmounting, a torsion member extending along the axis of symmetry of saidfork and being secured to the bridge means, a reaction mass secured tosaid torsion member, means for driving said fork to cause said tines tovibrate substantially at their resonant frequency and sensing meansisolated from the mounting for sensing the vibratory motion of thereaction mass to provide a measure of the input rate about the axis ofsymmetry of said fork. .Iaddend..Iadd.
 11. A system as in claim 10wherein said sensing means is isolated from the mounting by the bridgemeans. .Iaddend..Iadd.12. In a vibratory angular rate sensing system, asupport structure, a driven fork lying in a plane and having an axis ofsymmetry, said driven fork having first and second tines having at leasttheir outer extremities spaced apart, a torsion member having first andsecond ends with the first end being secured to the fork and with thetorsion member extending between the first and second tines, a masssecured to the second end of torsion member, said driven fork, saidtorsion member and said mass forming a single unitary rotatablestructure to effect an interdependency between the driven fork, thetorsion member and the mass, connecting means connecting said rotatablestructure to said support structure, means coupling energy into saiddriven fork to cause vibratory motion of the tines of the said drivenfork in the plane of said driven fork, said connecting means includingmeans for isolating said rotatable structure from said support structureso that minimal disturbance is transferred from the support structure tosaid rotatable structure, and pickoff means mounted on the torsionmember and being substantially independent of vibratory motion of thesupport structure for sensing torsional oscillatory motion with respectto the driven fork to provide a measure of the input angular rate to thesupport structure about said axis of symmetry of said driven fork sothat vibratory motion of the support structure is of minimalsignificance in the measurement of the input angular rate..Iaddend..Iadd.13. A system as in claim 12 wherein said connecting meansis in the form of bridges. .Iaddend..Iadd.14. A system as in claim 12wherein said torsion member extends along the axis of symmetry..Iaddend..Iadd.15. A system as in claim 12 together with an additionalmass carried by the free end of each of the tines of the driven fork..Iaddend..Iadd.16. A system as in claim 12 wherein said tines of saiddriven fork extend at an acute angle with respect to each other..Iaddend..Iadd.17. A system as in claim 14 wherein said connecting meansis secured to said rotational structure at two spaced apart points..Iaddend..Iadd.18. A system as in claim 17 wherein said connecting meansis in the form of first and second bridge members. .Iaddend..Iadd.19. Asystem as in claim 18 wherein the mass secured to the torsion member isbeyond the outer extremities of the tines of the driven fork..Iaddend..Iadd.20. A system as in claim 12 wherein said supportstructure, said connecting means and said rotatable structure are formedof a piezoelectric material. .Iaddend..Iadd.21. In a method for sensingangular rate by the use of a support structure and a single unitaryrotatable structure connected to the support structure with therotatable structure comprising a driven fork having a major axis ofsymmetry, a torsion member, the steps of substantially isolating therotatable structure from the support structure so that minimaldisturbance is transferred from the support structure to the rotatablestructure with respect to relative motion between the support structureand the rotatable structure, supplying energy to the driven fork tocause vibratory motion of the driven fork in a plane of the driven forkand sensing on the torsion member the torsional oscillatory motion ofthe torsion member with respect to the driven fork with minimal inputfrom the vibratory motion of the support structure to provide a measureof the input angular rate about the major axis of symmetry of the drivenfork. .Iaddend.